This invention relates to an axially movable quill for a top head drive assembly of the type which moves along a mast of an earth drilling machine to support and rotate a string of down hole tubulars.
An increasing number of earth drilling machines are provided with top head drive assemblies. Generally speaking, such earth drilling machines include a mast which guides the top head drive assembly for movement along the drilling axis. Typically, the top head drive assembly is supported by means of cables, and it includes means such as an electric or a hydraulic motor for rotating a quill which supports and rotates a string of down hole tubulars. U.S. Pat. No. 4,314,611 discloses one such top head drive assembly. In this application the term "down hole tubular" is intended in its broad sense to encompass the full range of tubular elements used in earth drilling and related activities, including drill pipe, drill casing and the like.
In tubular handling operations the quill or an adapter sub threadedly connected to the quill must be mated with the box end joint of a length of down hole tubular. The quill or adapter sub is lowered onto the box end joint, and then, as the quill is rotated by the top head drive assembly, a threaded connection is made up with the down hole tubular. In the event the top head drive assembly is not positioned properly, the box end joint of the down hole tubular can be damaged by excessive forces applied to the box end joint by the top head drive assembly.
In order to overcome this problem, it has been conventional practice for some time to mount a telescoping thread saver sub between the quill and the upper joint of down hole tubular. The thread saver sub is designed to telescope through a predetermined range of travel in response to compressive forces. The thread saver sub acts to limit the forces that can be applied by the top head drive assembly to the down hole tubular, at least until the range of travel of the thread saver sub is exhausted.
Though conventional thread saver subs can operate to protect the threads of the down hole tubular from damage, they suffer from the disadvantage of potential catastrophic failure in a blowout. If the drilling mud rises in the bore hole during a drilling or casing operation, large pressures can be applied to the seals of the thread saver sub. Any failure of these seals can cause the thread saver sub to fail catastrophically.
The present invention is directed to improvements to a top head drive assembly which largely overcome these disadvantages by mounting the quill itself for axial movement in the top head drive assembly.